Municipal Darwinism is the technological ecosystem by which most of the world of Mortal Engines works. While Shakespeare is referring to humans as being mortal engines, Reeve's turns the two words on the head of what Shakespeare meant.

The larger predator cities consume smaller cities for their resources. Physical resources are used for fuel or re-utilised. Humans living on the captured cities can be enslaved or eaten.

The main theory of Municipal Darwinism is a predator and prey cycle; if the bigger town is faster than the smaller, the smaller town will be eaten.

But if the smaller town is faster than the bigger town, the bigger town risks running out of fuel and thus losing it's prey or even facing attack itself in a reversal of fortune.

While in the context of the book's universe this form of Darwinism has existed for 1000s of years since the 'Sixty Minute war', it's a zero sum game which refers to the fact that the society that engages of Municipal Darwinism is not actually a sustainable means of living.

All the cities' engines are indeed mortal as eventually there will be nothing left to consume and they will fail and die.

Readers familiar with Reeve's work will know that he's a bit of a literary magpie and nicks the odd line from a song here and there or a book or line from a classic play to liven up his books. He does it really well - so well we suspect that a lot of the younger readers he has will miss many things he does!